Radio-frequency carrier-signal limiting system



Feb. 26, 1946. l.. FLcUR-rls 12,395,615

RADIO-FREQUENCY CARRIER-SIGNAL LIMITING SYSTEM I Filed oct. 2a, 1941 2sheets-sheet 1 L. F. CURTIS Feb. 26,1946.

vRADIOLFREQUENCY CARRIER-SIGNAL LIMITING SYSTEM Eiied oct'. `2e [1941 2sheen-sheet 2 93m w r N o s Se: Siam, QwNm 9.@ o w- INVENTOR L E Lis F.GuRTls` ATTORNEY 2,395,615 RADIO-FREQUENCY CARRIER-SIGNAL tric-s GSYSTEM Leslie F. Curtis, Great Neck, N. Y.,

Hazeltigie Corporation, a corporation Ware assigner to of Dela-Application OctoberA 28, 1941, Serial No. 416,851

' 14 claims. (o1. 17a- 44) The present invention relates to an improvedcarrier-signal limiting system and, particularly, to an improvedlimiting system of the type which utilizes multi-electrode vacuum tubesand accomplishes limiting action by operation of the tubebeyond'anode-current cutoff during a portion of each carrier-signalcycle.

Of the numerous types of limiting systems heretofore proposed, theself-biased vacuum tube type has perhaps the most promise by virtue ofsuch outstanding advantages as its rapidity and uniformity of limitingaction, 4its simplicity and ease of adjustment, itsself-biasing featurewhich eliminates the necessity of xed biases with their Y atten tdisadvantages, and its freedom from the requirement of any form Theprior art limiting systems of this nature have employed a single vacuumtube having a control electrode connected to aninput circuit including agrid condenser and grid leak to develop its own control-electrode biasby peak rectification of carrier signals translated through the system.

of neutralization.

Limiting occurs on all carrier signals of intensity greater than thatrequired to bias the vacuumtube to cutoff.

Such single-tube limiting systems of the prior art have the disadvantagethat limiting is delayed in time more than is desirable due in largemeasure-to the fact that the grid provides only half-wave rectication,Aa limitation which reduces the speed with which the bias potential canbe developed. This delayed action is also due in partto the fact thatthe values of the grid leak and grid condenser required for adequatesensitivity are necessarily so large that the time constant of thesystem is appreciable. Transient disturbances cannot, therefore, belimited as completely as are the translated carrier signals 'and theoutput of the limiter system is materially reduced over an appreciableinterval upon the application thereto of a large positive pulse oftran-` sient disturbance. Any attempt to decrease the time constantofthe grid condenser and grid leak impairs the sensitivity of the systemover itsl entire` rangeA of operation and increases the value of signalinput at which limiting begins. There' is the additional disadvantagethat single-v tube limiters of this nature-do not have balanced inputand.. output circuits, whereby even-order harmonics which occur byvirtue of the'limiting actlonappear inthe output circuitof the limiterand cannot be balanced. out in the coupling between the system and thesucceeding amplifier or detector stage. e rv YIt is an object of theinvention, therefore,- to provide a new and improved carrier-signal'limiting system which avoids one or moreof the of the priorart devices.

It vis-a further object of the invention to pro.

above-mentioned disadvantages and limitationsy cuits connecting thetubes in vide a carrier-signal limiting system having a self-biascircuit of relatively short time constant in which is developed therequired self-bias'potential and one which, therefore, provides exltremely rapid limiting action.

It is an additional object of the invention io provide a carrier-signallimiting system having improved sensitivity over its entire' operatingrange and particularly the portion thereof just before limiting begins.

Inaccordance with one embodiment of the invention, a radio-frequencycarrier-signal limiting system comprises a pair of vacuum-tube repeatershaving predetermined operating characteristics, andindividual input andoutput cirpush-pull relation, the input circuits being adapted to haveradiofrequency carrier signals applied thereto and the output circuitsbeing highly selective against all harmonics of the carrier4 signalsapplied to` the input circuits. The system also includes means forlimiting to a predetermined value the maximum amplitude ofcarrier-signal output of the system comprising capacitance meanscomposed primarily of the input capacitances of the repeaters and thedistributed capacitances of the input circuits and an impedance commontothe input circuits for the magnitude of which variesin accordance withthe amplitudeof the applied carrier signals, the impedance being soproportioned with relation to the operating characteristics of thevacuumtube repeaters that the carrier-signal translating cess of thatnecessary to produce a self-.bias po tential of sufficient magnitude tobias the tubes e y to cutoff.

For a better understanding of the present invention, together with otherand further objects thereof, reference is had to the followingdescription taken in connection with the accompanying drawings, and litsscope will be pointed out in the appended claims.

Referring now cuit diagram, Apartly schematic, of 'a completefrequency-modulated carrier-signal receiver embodying the invention;Fig. 2 is a graph illustrating anode-current and control-electrodepotential relationships for each of the tubes and is used in explaining`the operation of the invention; Fig.. 3 represents steady-stateinput-output characteristics of the limiting system over its usefulrange of operation; Fig.'v 4 is a graph illustrating a method ofgraphical solution of certain equations derived in a mathematicallanalysis of the operation of the inventio'n; and Figs. 5 and thelimiting system ofA the invention.

developing a self-bias potential to the drawings; Fig. 1 is a cire oithe Fig;

6 are graphs represent- Referring now more Particularly to Fig. 1,lthere is represented 'schematically a complete frequency-modulatedcarrier-signal receiver .of a

conventional design embodying the present in-` vention in a preferredform. 'Ihe receiver includes a radio-frequency amplifier ill having itsinput circuit connected to an antenna system II, I2, and having itsoutput circuit connected to an oscillator-modulator I3. 'Connected incascade with the oscillator-modulator I3, inthe order named, areanintermediate-frequency ampliner Il of one or more stages, acarrier-signal limiting system I5, more fulLv described hereinafter, a

frequency detector and automatic amplication control systeml I6, anaudio-frequency amplifier Il of one or more stages, and a soundreproducer `the various units thereof unnecessary. Considering brieflythe 'l operation of the receiver as a whole, and neglect ing for themoment the operation of the limiting system I5, presently to bedescribed, a desired frequency-modulated carrier signal is selected andamplified by the radio-frequency ampliiier I0, converted to afrequency-modulated intermediate-frequency carrier signal in the oscil-vlator-modulator I3, amplified in the intermediate-frequency ampliierI4,l limited to a predetermined substantially constant amplitudeA by thev limiting system ligand detected by the frequency detector |'5,therebyto derive the audiofrequency modulation components. The audio- Y`frequency components are, in turn, amplified in the audio-frequencyampller I'l land are `reproduced by the sound reproducer i8 vin aconventional manner. The automatic amplincation control bias developedbythe automatic amplification control system ofunit i6 is effective tocontrol the ampliilcation of one or more of the units I0, I 3 and 'I4 tomaintain the signal input i. lto the limiter I5 within a relatively`narrow range for a wide range of received signal intensities.

y Referring now more particularly to the portion of the system embodyingthe present invention, the limiting system I5 comprises a pair ofpentode-type vacuum-tube repeaters `I.9,'2|l having predeterminedgrid-cathode rectincation characteristics and predetermined inputvoltage-output current operating characteristics providing 'relativelysharp anode-current cutoi. The penjtode type ofvacuum tube is, as iswell known, one which includes a screen electrode between `its controlelectrode and its anode electrode for reducing the capacitance betweenthe last-mendenser 2l to the mld-frequency ot the range of frequencydeviation of the carrier signal applied to the transformer 22. Thefrequency response of the tuned circuit 2|, 22 is broadened by resistors24, 24 winch arel connected across individual halves oi' the transformersecondary winding 2i. There is provided means for limiting to apredetermined value the maximum amplitude ofv carrier-signal output ofthe system comprising capacitance means composed primarily of the ingridrectlcation of the applied carrier signal.

The input electrode capacitances of tubes I9 and 2l are represented bydotted-line condensers 26 and 21, respectively, and the capacitance toground of the center-tap of the secondary winding 2I by the condenser28', although thesecapacitances may be comprised in part by thedistributed capacitances of the individual input cirf cuits.

The output electrodes of vacuum-tube repeaters I9 and 2|! are connectedin individual output circuits comprising individual halves of theprimary winding 28 of an output transformer 29 coupled to the frequencydetector I6 -ina` conventional manner. The primary winding 28 is tunedby a condenser 30 to the mid-frequency of the range ,of frequencydeviation of the carrier signal applied to the limiting system. and thefrequency-response characteristic of the tuned circuit thus formed isbroadened by the resistors 3|, 32 winch are connected across individualhalves of the transformer primary winding l2l. Vacuum tubes I8 and v2l)are suitably energized circuits. Assume now that a lrelatively weakcarrier signal is applied to the input circuits of the tubes. Thecontrol grid oi each tube rectifies individual half-cycles of the`carrier signal to develop across the resistor 25 a unidirectionalnegative-bias potential. This self-bias potential,

-' lwhich charges the inherent input-circuit capactionedelectrodes. Eachof tubes ,I9 and 20 thus includes/means for reducing the capacitivecoupling between lthe input and output electrodes thereof, the screenelectrode comprising means for efiectivelydecreasing' the translation ofcarrier signals directly from the input to the output circuits of therepeater tubes by interelectrode :capacitive coupling through thevacuumy tube repeaters. The input electrodes of vacuum tubes I9 and. 20are connected in individual input circuits comprising individual halvesof the secondary winding 2I- of an input transformer 22. The secondarywinding 2l is tuned by a conitances represented by the condensers 26,2B' and 21, increases in magnitude until the control grids are biasedsuiilciently negatively that peak rectification o'f the applied carriersignal occurs, which usually takes plac in a few carrier-frequencycycles. i A

Referring now to Fig. 2,-which represents the 'operating characteristicof either of thetubes lil or ,20, the axis` ,-Il is the zero grid-biasaxis of the characteristic while the axis a-a represents -th'e axis ofthe carrier signal'applied to the control grids of the tubes, this axisbeing displaced from the axis Il-h by an amount equal to the bias Ve1developed across the resistor 25 by peak rectiilcationof the appliedcarrier. signal reprecharacteristic is not exceeded, the carrier signalis amplified in the hunting system. I5 and appears v in the outputythereof with substantially undis-r from a source of space current 33.The individsion of the resistor 25 in torted wave form as represented bycurve c. The time constant of the input circuit comprising the inherentcapacitances 2B, 26', 2l and the resistor 25 is of the order of two orless microseconds,`.for example; or in terms of frequency is of theorder of several cycles, for example, 3 to 5 cyclesfor an appliedcarrier signal having a frequency of 3 megacycles, so that for allpractical purposes the bias developed across the resistor 25 may beconsidered a constant bias over any given carrierfrequency cycle, asrepresented by the linear axis steady-state translation characteristicof the limiting system of Fig. 1 with the resistor 25 propor tioned inthe manner presently to be described. It will be observed from this gurethat the output of lthe limiting system increases rapidly to the pointm, at which limiting begins, and that v the output is thereaftersubstantially constant Assume now thatv the carrier wave applied to thelimiting system increases in amplitude. The control grids of tubes. I9and 20 again peakrectify the carrier wave to develop acrossresistor 25alarger self-bias potential. Thus a carrier signal having an amplituderepresented by curve d produces a self-bias potential of magnitude e2 tobias both tubes beyond cutoff during a portion of each carrier-signalcycle, whereby only a portion of each positive half-cycle ofA theapplied carriersignal is ampliied and appears in the output circuits oftubes I9 or 20, asrepresented by curve e. Since tubes I9 and 20 areconnected in push-pull relation, the individual outputs of' the tubeshave a phase difference of 180 degrees proper choice for `the value ofresstor` is,v o f for increasing intensities of carrier signal applied-tol the limiting. system. The' steep slope of the portion o, m of thiscurve indicates that the limiting system has a high ampliiication forcarrier signals of suiiiciently low intensity that limiting does notoccur, and that limiting begins.\at a relatively low amplitude of thecarrier-signal input, both of which are highly desirable characteristicsof a limiting system. lf the value of the resistor 25 is chosen toosmall, the translation characteristic of the limiting system continuesto rise for all values of applied carrier-signal amplitude, asrepresented by curve y. Irthe resistor 25 is too large, the output ofthelimiting system increases until begins and thereafter rapidlydecreases,`"as" represented by curve z. The

course, that which renders the output of the limiting systemsubstantially constant over the major portion of its translationcharacteristic.

and there is applied to the output tuned circuit,

l 28, carrier signals having the wave form represented by the combinedoutput signal curves e, e' for the two tubes `in push-pull. The tunedcircuit 28, 30 selects'only the fundamental-'frequency component,represented by broken-line curve f, and applies this component throughthe output transformer 29 to the detector of unit I6. Should the'carriersignal applied to the input circuits of tubes .I9 and 2li increase still'further in amplitude, as represented by curve g, a larger self-biaspotential es is developed across resistor 25 and tubes I9 and 2Ii` arebiased even further Actually, the steady-state translationcharacteristic of the system depends not only upon the value of theresistor25, but additionally upon the control grid-cathode conductancecharacterv istic of the tubes I9 -and 20 and on the control- 35mathematical analysis of the operation ofthe l beyond cutoff. Thereappears in the output circuit ofeach tube correspondingly smallerportions of positive half-cycles of the carrier signal as represented bycurves h, h'. The fundamental component Ic of the output wave h, h' ishere again selected by the tuned circuit 28, 30 and applied to thedetector of unit I6.

The value of resistor 25 is so proportioned, in a manner presently to beconsidered in greater detail, in relation to certain operatingcharacteristics of the vacuum tubes I9 and 20 and the capacitances ofcondensers 2,6, 26' and 21, that the fundamental component of theresultant'carrier signalsdeveloped in the output circuits of tubes I9and 20 is substantially constant in amplitude for all values of appliedcarrierl signals in excess of that necessary to produce a'self-biaspotential of sufiicient magnitude to bias the tubes to cutoi during atleast a portion of eachcarri'ersignal cycle. By proportioning resistor25 in this manner, the steady-state carrier-signal translathe'developedgrid bias and the point of clipping the common portion of grid biasvoltage -necessary for anode-current cuto. This will 'beeclear from'thefollowing limiting system.

The control-grid current of each of the tubes- I9 and 20 is expressed bythe following relation:

. ig=Aeg3/2' (1) where:

V`1A=a constant derived from the actual grid voltage-grid currentcharacteristic of the particular tube used and is equal to the gridcurrent for a grid potential of one volt; and

eg=the instantaneous positive control-grid cath-- ode voltage.

termined by the following relation:

' pagaran-en where Um=the transconductance of eitherfof the I' 'I9 and20;.and Ec=the` control-grid bias rent cutoi.

since tue-controlada unscathed lements or tubes I9 and 20 act asa diodere constant circuits, n Y 26', .21 and the resistor; 25, develop asteady bias I r, the time between the-grid and cathode elements asprevi- I ously pointed out, whereby the voltage eg which.

produces grid current has a wave shape corresponding to the clipped peakof a cosine wave.

being determined by the. vfollowing relation:

E=the peak value ofthe applied signal voltage; 6=wt and a =the angle,having plus andminus values, dur--V ing which4 grid current flows. A

The anode current lof each of these tubes is denecessary for plate cur--prislng the condensers 26,

a=sral=ieee a-eee i5/1 ai The man grid current dui-ing eeen e'yeie oftheapplied signal voltage. isdetermined by integration between the angles-a and +o of the gridrelation:-

where: A

R=twice the value of the resistor 25 since the latter resistor iseffectively two resistors in parallel. one of which is included in theinput circuitof vacuum tube I8 and'the'other in the ,input circuit ofvacuum 'tube 2l insofar as the present mathematical analysis isconcerned.

combining Equations '5 Asind o gives the relation:

Y 1=' IAE 3i/2(1-eoe ai Theplate current flows over a'larger angle bthan tire arid current as determined by the ex ri: is convenient toutilize a parameter P, where: `ieri/iii, (9)

"ijhenEquationSmaybewritten: -h

The instantaneous plate current the angles-b and -i-b has the value:

' =aiPse eoe o-eeeti (1 1) Since cne'umitersysiemhasenmea oimiuceircuit,only the fundamental component of this plate currentis useful indeveloping a steady-state voltage acro the output circuit. The peakvalue' of thiscomponent, which is determined by integration of Equationil between the angles '-b' and +I. hasavalue by the relation:'I,.=g.;E.:(beee bein 6)' The steady' Leiste tram ung i:iiartiste-istievof eachhalfoftbelimitingsystem,comprisingone u co of under theassumption that the amplitude of the of the tubesil or' Il. may bederivedby simultaneous solution of vEquations 'i'. 10, and l2. A

general mathematical! solution of these equations' .is not feasible buta graphical solution may readily'. be had. Thus', referringto Fig.' i,ciirvea A, fB andcare obtained byplotting coso, calculated from Equation7, against P (plotted conveniently as relative Vinputin decibels) forassumed values of'RAEelf. CurvesD, E and F are obtained by plotting cos.b, calculated from Equation ifor thesame-limitsofeos a, against?.CurveGis denvedbyniottinstnenmeuon "-'missin asoaoisFrom'Equatlonsland3.thevalueoftheinstantaneousgrldcurientmaylieexpreiisedbythelcosa (z).

in rie.; by meramente: am veroeei construetion lines, the arrow headsindicating the order of procedure from one set' of curves to another.

In this graphical' solution, it is convenient to plot bothP and f lw-eoe bein bi to allogarithmic or, decibel scale. Root-meanl0 squarevalues of the fundamental component of theplatecurrentare. l y Ecgg.

multiplied by the ordinates of relative output.

The root-mean-square'values of the input voltage Imultiplied by theordinates of relative output.

It has been found that values of RAEeU? between :i5- and 40 give asubstantially constant or uniform steady-state input-output translationcharacteristic of the limiting system for values of P from 0.6 to 10whichis equivalent to a decibel rangeof-4iioi- 20 db.v i The actualnoise disturbance appearing in the audio, output of thefrequency-modulated carrierm wave receiver depends upon the nature ofthe interfereneethe band width andymmetry of the amplifier stagesincluding the audio stages and any compensation forpre-eniphasis in' thelatter, as well as lupon the operatingeharacteristic of the limitingsystem. If no amplitude transients are translatedby the limiting system,the limitirm Y' action can be said to be perfect. No audio output isproduced by ampli de variations in a perfectly balanced frequencyetector in the absence of 40 frequency deviation, but during thedeviations accompanying desired frequency modulation sudden changes inamplitude produce noise output which depends on the product of theinstantaneous detector. The function of the limiting system is tomaintain the carrier-signal output amplitude asA constant as possible,and the amount of amplitulle variation of the carrier -signalas appliedto the frequenydetector is a measure of thecarriersignalappliedtothelimiting systemis -or decreased by a'-transient noise pulse. for example, a pulse yof rectangular wave,.Inthisanalysisitwillbe assumed thatthe changes of control-gridbias ofthe tubesl l! and 20 due tothe transient nolse'puise are suiiicientlyslow relative to the period ofthe carrier signal applied to thelimitingsystem that the grid cur- 'rent during the Vtransient; may beexpressed by Equation'. Thisequation is not exact when cos o isnegative, but this is'a condition not normally encountered in practiceso the error is not there- -viation and the relative change in amplitudeof the carrier signal applied to;the frequency de- *l enceintes fromminnen :messina eos o. 'me 'zo 'fore sisnincens. .As will be seen later,the signin- I' v'method of graphically solving Equations '1, Vlil-and'l2 by theuse of curves A to G. inclusive, to defXtermine any givenpointon the atealLv-'state t'aslation-characterisuc, 'represented by curve 3,foranyvalue'ofltAEel/Js indicated 15 trol-grid current as a fimctlon ofthe angle al cant intervals are those `when the control-grid Equation 5expresses the magnitude of the conaseaeis during which grid current`hows. The magnitude of the grid current may 'also be expressed by thefollowing relation:

Where I ef-the instantaneous magnitude of grid bias dur- `ing the'period of the transient amplitude change of the carrier signal appliedto the limiting system; R=the value of the resistor 25; and C=the totalcapacitance in sh t to"the resistor l 2'5, this capacitance equaling thesum oi the f capacitance of condensers 25,526' and 21.

Combining Equations 5 and`13 givesthe ex- K1=a constant depending ontheinitial \condi tions when time t=0. Y

Equation may be simplied into the forma:

vlimiting system may be vas follows:

1. Grid bias greater' than Erl-Ec During this phase, grid current andplatecurrent of vacuum tubes i9 and'20 are both cut ofi.

There is no audio output whatsoever except for'- the decay transients inthe circuits following the limiting' system, which transients are notfurther considered.

2. Grid bias greater than Ea but less than Eri-Ea During this phasethere is no grid curent,-but

the plate current increases with time as the grid bias decays. The decayof grid bias during phases 1 and 2 is determined only by the timeconstant RC of the control-grid circuit t FE# Re 3. Grid bias less thanEa During this phase, the grid bias decays according to Equation 20 andthe plate current continues to increase until the steady-state conditionis reached.

When the amplitude of the carrier signal applied to the limiting'systeml5 is suddenly in- During the Y growth of the control-grid bias fromlzero at time t=0, the specific value oi Kx is;

During the decay onthe control-grid bias from.

creased, as by a positive noise transient, the control-grid biasincreases according to Equation 18 until the steady-state condition isreached. The plate current of the tubes i9 and 2t initially assumes alarge value and decays to the 'steadystate value.

The plate current during any of these'phases may be determined bycalculating the angle of plate-current'ow corresponding to cos a' (orcos a") vfrom Equation 10, remembering that vcos a' and cos b' now varywithtime. Thus y or the calculated lvalue of eos a is greater thantransient unity. When the calculated. value of cos b' is greater lthanunity, plate current is cut off.

rent is e@ -from an adaptation of Equation 12 and where P is the valueof En/Ea following the application of the transient noise pulse to theapplied carrier `signal lIlle broken-line curve 'J of Fig. 5'represents' the change o f control-grid bias when the 'carrier signalapplied to the limiting system I5 has vthe plitude-modulationcharacteristic represented by thesolid-line curve K. there beim! anegative nents.- one plotted from data calculated from .Equation 18 forthe condition of a positive increase oi carrier-signal amplitude, asduring the intervals te to ta using one set of values for the dividedinto three parts The fundamental component of the plate curl pulseappearing during the interval t1 -to ti and va' positive noise impulse.during the interval from t: to t4. CurveJ has three compoconstants oiEquation 18 and is to t4 using another lset of values for the constantsoi' this equation. one component from Equation 22 during the period oi'decreased amplitude of the applied carrier signal, as during the periodti to t: and to ts using the proper values for the constants of Equation22 for these periods, and a third portion plotted from data calculatedfrom Equation as during the period of time from ts to iniinity. Thecircuit constants of the limiting system I B, in deriving data for curveJ, were assumed to have the following values:

. R=50,000 ohms.

Capacitance in shunt to the resistor -=total capacitance. of condensers26, 26' and 21=40 micro-microiarads.

Time constant=2 microseconds. s

The broken-line curve L of Fig. 6 represents the variation of platecurrent oi.' either of vacuum tubes' I9 or `2li when the amplitude ofthe car- Y very small time constant of itsinput circuit. This.

rier signal applied to the limiting system i5 l changes in the mannerrepresented by curve K oi Fig. 5. Curve L is plotted from datacalculated from Equation 24 when the circuit' constants oi" the limitingsystem I5 have the values just stated. In order to compare the transientlimiting characteristic of the limiting system of the present inventionwith a prior art type of limiting system similar to that of the limitingsystem il except that only a single vacuum tube is used therein, thebroken-line curve M of Fig.'5 and the broken-line curve N of-Fig. 6represent the change ot control-grid bias andthe change of platecurrent' of this prior art type of limiting system when the amplitude ofthe carrier signal applied to the limiting system varies in accordancewith curve K oi Fig. 5, the circui-J constants oi the limiting systembeing assumed to have'the following values: v

Self-bias grid resistor=l00,000 ohms. 4 1

Capacitance in shunt to. self-bias grid resistor= 250micro-microfarads.

Time constant=25 microseconds.

The curves L and N of Fig. 6 have perhaps. the greatest .y comparativesign'iiicance since the change of platey current of the 'limiter tube isa.- direct measure of the output of the limiting systenr and thus is ameasure of the transient-noise disturbance applied tothe-irequeiicydetecto whichiollows the limiting system.` From curve L, itwill be evident -that the output of the limiting system I! of thepresent invention is reduced to zero during the time tito tz when theapplied carrier signal has zero amplitude, has a sharp peak ofrelatively low amplitude and extremely.

short duration .immediately subsequent to the time is, has a muchlargerpealr amplitude of :isomers at the time aand does not resume itssteadystate output until approximately 65 microseconds after the'amplitude of the carrier signal is again normal at the time t4. Thetransient amplitudelimiting characteristic, represented by curveN, ofsuch prior art limiting system produces distinctly audible componentsduring the occurrence oi, andthe recovery from, transient noisedisturbances which may -be more serious than audible components producedby the original transient noise pulses without a limiting system. Theaudible components of the limiting system I5 ot the present invention,on the other hand, are very small. v'I'he improved transientamplitude-limiting characteristic oi the limiting system of the presentinvention is effected in large part vby the ltime constant is determinedby the resistor 2l and theminherent capacltances 20,' 26 and 21.Consequently, the transient amplitude-limiting characteristic of thellimiting system I5 is greatly improved by any reduction of the inherentca-` pacitances of its input circuit, there` being no need for physicalcapacitance in shunt to the resistor 25 as is necessary in the prior artlimiting system discussed, the characteristic of which is represented bycurves M and N of Figs. 5 and 6 respectively. l

The limiting action of the limiting system il is' substantiallyindependent ol the anode potential-'of tubes I8 and Il as long as theanode potential is above that required for distortionless vtranslationof relatively weak carrier signals. By

virtue of the use of a pentode type of tube which operates essentiallyas a constant current or curv` rent regulated device the limiting actionis also substantially independent of the anodeload, which aiiects onlythe gain of the limiting system for carrier signals of intensity belowthat atwhich limiting occurs.

As illustrative oi.' a speciiic' embodiment of the invention, thefollowing circuit constants are given for an embodiment oi the inventionshown' short duration immediately after` the carriersignal amplitude hasincreased at the time ts. and has zero output for a short time intervalof V approximately 3 microseconds immediately after thecarrier-signalamplitude is reduced to normal value at the time t4. In comparison, theprior art limiting system has a small peak amplitude over an appreclablylarger interval immediately f .after the time t2, has another whichpersists over substantially the entire interval ta to t4 after theamplitudeof the carrier signal is increased/jas byl a positive transientnoise pulse, .is completely cut ci! for an interval of 35 microsecondsafter the carrier-signal amplitude is reduced to normal 3 megacycles Itwill be evident from the above description of .the invention that acarrier-signal limiting sys- 'tem embodying the invention has theadvantages,

ilrst, that the system has improved gain over its entire translationcharacteristic and especially over that portion of its characteristicprior to the occurrence of limiting, and, second, that the magnitude ofthe selfbias potential of the system increases rapidly-with increasingvalues oi carrier-signal intensity, whereby limiting begins atV lowvalues of carrier-signal input amplitude.

The limiting system has balanced input and output circuits so thateven-order harmonics of the translated carrier signal are balanced outin the coupling to the succeeding amplifier or detector stage. 'I'hesystem is extremely eilective in limiting transient noise disturbances,since the time constant ofthe circuit across which the selfbiaspotential is developed is relatively'short, be-

ing only of the order of a few microseconds or,

in other words, a few cycles of the carrier signal..

Transient noise disturbances of either polarity are eectively limited bythe system. The limiting system of the invention has the additionaladvantages that it is of simple. circuit arrangement, is easily adjustedfor proper limiting action, and its operation is stableand substantiallyindependent of changes of anode energization andv anode loading. y

While there has been described what is at presof this invention, it willbe obvious to those skilled in the-art that various changesandmodiiications may be made therein withoutdeparting from the invention,.and it is, therefore, aimed 3. A radio-frequency carrier-signal.limiting system comprising, a pair of vacuum-tube repeaters havingpredetermined operating and relativiy sharp anode-current cutoffcharacteristics. individual input and output circuits connesting saidtubes in push-pull relation, said 'inis put circuits beingadapted Atohave radioffrequency carrier signals applied thereto and said outputcircuits being', highly selective. against all harmonics of the carriersignals applied to said input circuits, and means for limitingto apreent considered to `be the preferred embodiment l5 in the appendedclaims to cover all such changes and modications as fall within thevtrue spirit and scope of the invention.

What is claimed is: l. A radio-frequency carrier-signal limiting systemcomprising, a pair of vacuum-tuberepeaters having predeterminedoperating1 characteristics, individual input and output circuitsconnecting said tubes in push-pull relation, said input circuits beingadapted to have radio-frequency determined value the maximum amplitudeof v carrier-signal output of said system comprising capacitance meanscomposed primarily 'of the input capacitances of said repeaters and thedistributed capacitances of said input circuits and an impedance commonto said input circuits for developing a self-bias potential themagnitude oi which varies in accordance withthe amplitude -of theapplied carrier signals, said impedance being so' proportioned withrelation to said operating and anode-current -cutoii characteristicsthat the carrier-signal translation characteris tic of `said-system issubstantially constant ior values of applied carrier signals in excessoi that .necessary to produce a self-bias potential oi' sumcientmagnitude to bias said tubesto plate-cur,

carrier signals applied' thereto and said output` 30 termined value themaximum amplitude'of ca rrier-signal output of said system comprisingcacapacitances of said repeaters and the distributed capacitances of.said input circuits andan impedance common to said input circuitstor-developing a self-bias Apotential the magnitude of which -varies inaccordance with the amplitude of the applied carrier signals, saidimpedance being so proportioned with relation to said operatrent cutoff.l 4. A radio-frequency carrier-signal limiting system comprising. a'pair of vacuum-tube' repeaters having'predetermined operating and grid-lcathode recticatio'n characteristics, individual l input and outputcircuits connecting said tubes vin pushpull relation, said inputcircuits being adapted to have radioefrequency carrier signals appliedthereto Vand said output circuits; being highly selective against allharmonics of the car-- rier signals applied to said input circuits,and

means f orlimiting to a predeterl'nined.value'the maximumamplitude ofcarrier-signal output'of said system comprising capacitance meanscomposed primarily of the input capacitances oi said repeaters and thedistributed capacitances oi' vsaid ing characteristics that thecarrier-signal translation ^characteristic of said system issubstantially constantl for values of applied carrier signals in excessof that necessary to produce a selfbias potential of sufcient magnitude'to biassaid 2. A radiirequency carrier-signal limiting systemcomprising, a pair of vacuumatube repeaters having predeterminedoperating characteristics, individual input and output circuits conputcircuits being adapted to have radio-fre;

output circuits being highly selective against all input circuits and awimpedance common to said input circuits for deriving by gridrectiiication of said applied carrier signals a self-bias potenv tialthe magnitude of which varies in accordance v with the amplitude oftheapplied carrier signals, o said impedance being so proportioned withrelation to said operating and grid-cathode rectiiicationcharacteristics that the carrier-signal translation charsacristiel orsaidsysiem is substanvvtially constant Ivor values ofapplied carriersig-l necting said tubes in push-pull relation, Vsaid in, miv

input circuits, and means for limiting to a pre- 'in determined valuethe maximum amplitude o! carrier-signaloutpuof said system comprisingcapacitance means composed primarily of the input capacitances oi saidrepeaters and the distributed/ self-bias potential the magnitude ofwhich varies in accordance with the amplitude of applied carriersignalsgsaid resistor 'being so proportioned with relation to saidoperating -characteristlcs of said system is substantially constant forvalues of applied carrier signals in excess o! that neces-1v sary toproduce a self-bias potential of sufiicient `magnitude to bias saidtubes-to cutoff.

' capacitances of said-input circuits and a resistor, F5-

nals in excessof that necessary'to produce a selfbias potential ofsutlcient` magnitude to bias said quency carrier signals applied theretoand said tubes to wma' l 5. A 'radiofrequency carriersignal limiting I.ste com risin', aair of vacuum-tube re- ,harmonics of the carriersignals applied to said`v sy m p g p 'being `adapted to-haveradio-frequency carrier signals applied thereto andsaid output circuitsbeing highly selective against all harmonics of the'carrier. signalsapplied to said input circuits.

and means for limiting to a predetermined value the maximum amplitude of'carrier-signal output "of saidsystem comprising capacitance means ycomposed primarily lof the input capacitances of that the carriersignal(translation characteristic said repeaters andthe distributedcapacitances of said input circuits and an impedance common .15 biaspotential the magnitude ,of which varies in carrier signals, saidimpedance being so propor- 'tioned with relation to saidcharacteristicthat the carrier-signal translation characteristic of said system vissubstantially constant for values oi applied carrier signals in excessof that necessary to produce a self-bias potential .of sumcient fmagnitude to bias said tubes to cutoff,

teristics and predetermined input circuit capacitanccs, individual inputand` output circuits connecting said tubes in push-pull relation, saidinput circuits being adapted tohave radio-freoutput `circuits beinghighly selective against all harmonics of the carrier signals applied tosaid input circuits, and means for limiting to a pre-` determined valuethe maximum amplitude of can-ier-signal output of said system comprisingcapacitance means composed primarily of said input`capacitanoes oi' saidrepeaters and the disltributed capacitances of s aid input circuits andan impedance common-to said input circuits for developing a self-biaspotential Atheuiagnitude of which varias in accordance with theamplitude oi the applied carrier signals, said impedance beingproportioned with relation to said capaci- 8 4 A l asesora accordancewith thc amplitude ci the implicav c cnc vcit, inuividutiinnutnndcutputcircuits ccnnecting said tubes in push-pull relation, said inputcircuits being adapted tov have radio-frequency carrier signals appliedthereto and said s output circuits being nigniy 'selective against nuharmonics ofthe carrier signals applied to' said input circuits, andmeans for limiting to a predetermined value the maximum amplitude of'a-.i'esistor R common td'said input circuits for developing a 'self-biaspotential the magnitude oi which varies in accordance with the amplitudeof applied carrier Signals, Said resistor being so proportioned withrelationV to said cutoii' bias Ec tand said constant'A that -the productRAEcll is quency carrier signals applied thereto and said substantiallybetween 35 and 40, whereby-the carrier-signa1 translation characteristicof said system is substantially constant for values of f applied carriersignals from 0.6 to Iltimes saidv bias Ec.

' currentv to cutoiI-and the control-electrode current is A e331 wherees is the instantaneous positive voltage applied to the controlelectrode and A is a constant equal to the value o1' control-electrodecurrent for-a control-electrode potential of tances of said limitingmeans to cause said lim' 3o one volt, individual input and outputcircuits iting means to have a time constant of the order of only ai'ewmicroseconds, whereby the carrier` connecting said `tubes in push-pullrelation, said input circuits being adapted to have radici-irequencycarrier signals applied thereto and having predetermined inputcapacitances and said outat put circuits being highly selective againstan nar- 1 monies o! the carrier Signals applied to said input circuits,and means for limiting. to a predetermined value the maximum amplitudeof carriersignal output of said system comprising a resistor -peatershaving predetermined operating charac- 40 R. commento said inputcircuits for developing a teristics and predetermined input circuitcapacitances, individual input and output circuits connecting. saidtubes in push-pull relation, said linput circuits beingadapted to haveradio-frequency carrier signals applied thereto and said 4 5 stant Athat the. product RAEcl/2 is substantially self-bias potential themagnitude of which varies .with reiaticnto said cutcfr biss Ec andscudccnoutput circuits being highly selective against all harmonics ofthe carrier signals applied tosaid 5 1 input circuits, and means forlimiting tti a. p determined value the maximum amplitude of capacitance'means composed primarily ofvsaid input capacitances of said repeatersand the distributed capacitances .of said input circuits and animpedance common to said input circuits for developing a self-biaspotential the magnitude of which varies in accordance with the amplitudeof the applied carrier signals, said impedance being proportioned withrelation to said capacitances of said limiting means to cause saidlimmicroconds, whereby thevcarrierasignal translation characteristic ofsaidl system isl substantially 'c'onstant for values of applied carriersignals in excess of that necessary to produces self- `is a constantequal to the value of control-electrode current i'or a control-electrodepotential ot between and 40, whereby the steady-state car. riet-signaltranslation characteristic of said systemis substantiallyI constant forvalues of applied e. carrier signals from 0.6 to 10 times said bias Ec,carrier-signal output of said system comprising said input circuitvcapacitances being sufficiently small with relation to the value ofsaid resistor that-said capacitances and said resistor have a `timeconstant of the ordero! only a few microseconds, thereby to minimize theeiect in said- 55 output circuits of undesired transient amplitudevvariations`of the applied carrier' signals.

10. A radio-frequency carrier-signal limiting v system comprising, apair .of vacuumg-tube l peaters having predetermined operatingcharaciting means to have a time constant less than two 6 terlstics,individual input and output circuits connecting said tubes in push-pullrelation, said input and output circuits being' tuned to the meanfrequency ct radio-frequency carrier signals applied to said inputcircuits and said output cir-i- '4 bias potential of suiiicientmagnitude to bias said 'cuits' being. highly selective against allharmonics of the carrier signals applied to said input circuits, andmeans for limiting t0 a predetermined value the maximum amplitude ofcarrier-signal output of said system' comprising capacitance 7 meanscomposed primarily `of the input capacitances of said repeaters andthedistributed ca-v pacitancespf said vinput circuits and an impedancecommon to said input circuits for developing a self-bias potential themagnitude of which varies in accordance with thc amplitude ci theapplied carrier signals, said impedance being so proportioned withrelation to said operating characteristics that the carrier-signaltranslation characteristic of said system is substantially consignaltranslationcnaracterisucor said'sysm is substantially constant forvalues of applied carpeaters having predetermined operatingcharacteristics, individual input and output circuits con necting saidtubes in push-pull relation, said input circuits being adapted to haveradio-frequency carrier signals applied thereto and said output circuitsbeing highly selective against all harmonics of the carrier signalsapplied to said Y input circuits,lmeans for effectively decreasing thetranslation of carrier signals directly from Asaid input to said outputcircuits by inter-electrode capacitive coupling through said vacuumtuberepeaters, and means for limiting to 'a predetermined value the maximumamplitude ofcarrier-signal output of said system comprising capacitivemeans composed primarily of the input capacitances of said repeaters andthe distributed capacitances of said input circuits and an impedancecommon to said input circuits for developing a self-bias potential themagnitude of which varies in accordance with the amplitude of theapplied carrier signals, said vimpedance being so proportioned withrelation to said operating characteristics that .thecarrier-signaltranslation characteristic Aoi said system issubstantially constant for values of applied carrier signals in excessof that necessary to produce a self-bias potential of suiiicientmagnitude to bias said tubes to cutoff.

12. A radio-frequency carrier-signal limiting system comprising, a pairof vacuum-tube repeaters having predetermined operating characteristicsand including means for reducing the capacitive coupling between theinput and output electrodes thereof, individual input and outputcircuits connecting said tubes in push-pull relation, said inputcircuits being adapted to have radio-frequency carrier signals appliedthereto` and'said output circuits being highly selective against allharmonics of the carrier signals applied to said input circuits, andmeans for limiting to a predetermined value the maximum am- `plitude ofcarrier-signal output of said system comprising capacitance meanscomposed primarily of the input capacitances of said repeaters and thedistributed capacitances of said input circuits and an impedance commonto said input circuits for developing a self-bias potential themagnitude of which varies in accordance with the amplitude of theapplied carrier signals, said impedance being so proportioned -withrelation to said operating characteristics that the carrierrier signalsin excess of that necessary to produce a self-bias potential ofsufficient magnitude to bias said tubes to cuto?.

13. A radio-frequency carrier-signal limiting system comprising, a pairof vacuum-tube lrepeaters having predetermined operating characteristicsand each including a screen electrode between the control electrode andanode electrode thereof for reducingthe capacitance between saidlast-mentioned electrodes, individual input and output circuitsconnecting said tubes in push-pull relation, said input circuits being.adapted to have radio-frequency carrier signals applied thereto andsaid output circuits being highly selective against all harmonics of thecarrier signals applied to said input circuits, and means-for limitingto a predetermined value the lmaximum amplitude of carrier-signal outputof said system comprising capacitance means composed primarily of theinput capacitances of said repeaters and the distributed capacitances ofsaid input circuits and an impedance common to said input circuits fordeveloping a self-bias potential the magnitude oi' which varies inaccordance with the amplitude of the applied carrier Signals, saidimpedance being so proportioned with relation to said operatingcharacteristics that the carriersignal translation characteristic ofsaidsystem is substantially constant for values of applied carriersignals in excess of that necessary to produce a self-bias potential ofsufficient magnitude to bias said tubes to cuto.

14. A radio-frequency carrier-signal limiting system comprising, a pairof pentode vacuumtube repeaters having predetermined operatingcharacteristics, individual input and output cir,- cuits connecting saidtubes in push-pull relation, said input circuits being adapted to haveradiofrequency carrier signals applied thereto and said output circuitsbeing highly selective against all harmonics of the carrier signalsapplied to said input circuits, and means for limiting to apredetermined value the maximum amplitude' of carrier-signal output ofsaid system comprising capacitance means composed primarily of the inputcapacitances of said repeaters and the distributed capacitances of saidinput circuits and an impedance common to said input circuits fordeveloping a self-bias potential the magnitude of which varies inaccordance with the amplitude of the applied carrier signals, saidimpedance being so proportioned with relation to said operatingcharacteristics that the carrier-signal translation characteristic ofsaid system is substantially constant for values of applied carriersignals in excess of that necessary to produce a self-bias potential oisufficient magnitude toebias said tubes to cutoff.

LESLIE F. CURTIS.

